All cellular communication systems are divided into cells, wherein user equipments (UE) are served by one base station or, when in soft handover, several base stations. Each base station may serve UEs in more than one cell. The position of a certain UE is given with respect to a specified coordinate system. A substantial number of positioning methods of varying sophistication have been suggested over the years.
Today, the cell ID positioning method, which determines the terminal position with cell granularity, represents the backbone low-end approach in the majority of the cellular systems. The method has the advantage of an instantaneous response and availability wherever there is cellular coverage. Due to the advantages of the cell ID method, attempts have been made to enhance its accuracy while maintaining the benefits. The most promising attempt is to augment the cell ID method with an assessment of the round trip time (RTT), i.e. the travel time of a data packet back and forth between the terminal and the base station. Together, this defines a truncated circular strip around the base station and has been denoted an ellipsoid arc in the document 3GPP TS 23.032 V.4.0.0, “Universal Geographical Area Description” (April, 2001) issued by the 3rd Generation partnership project (3GPP).
The dominating high-accuracy positioning method in almost all cellular systems is the Assisted Global Positioning Satellite (A-GPS) method. A-GPS is an enhancement of the global positioning system (GPS) and described, e.g., in the document “Understanding GPS—Principles and Applications” by E. D. Kaplan, Norwood, Mass.: Artech House, 1996.
FIG. 1 illustrates an example of an A-GPS positioning system that is implemented in a cellular communication system 100, which in this case is a WCDMA system. In this system, the radio network controller (RNC) 105 acts as the node that collects, refines, and distributes assistance data to the terminals 110 (denoted user equipment (UE) in WCDMA) over the RRC interface 104 and reference GPS receivers 112 over the interface 102. For this purpose, the node 105 is equipped with a GPS interface 107. The core network (CN) 115 requests and reports positioning of a UE 110 over the RANAP interface 120 and, in response, the RNC 105 may use various kinds of A-GPS techniques. All these techniques build, however, on assistance data to be handled by a node in the cellular communication system. The RNC orders positioning measurements to be performed in the UE whereby the corresponding measurements are performed by dedicated A-GPS receiver hardware in the terminals. These receivers detect GPS transmissions, e.g. GPS ranging signals 109, from the satellites 108 that are also denoted space vehicles (SVs). GPS reference receivers 112 are attached to, e.g., the cellular communication system for collecting assistance data that enhances, when transmitted to the GPS receivers in terminals connected to the cellular communication system, the performance of the GPS terminal receivers. Additional assistance data is collected from the cellular communication system directly, typically to obtain a rough initial estimate of the position of the terminal together with a corresponding uncertainty of the initial estimate.
In positioning applications, the positioning method used by, e.g., the Radio Network Controller in a WCDMA Radio Access Network (RAN), determines the region where the terminal is located, said region expressed as one of the geometrical shapes known from the above-mentioned document 3GPP TS23.032. The selected geometrical shape is most often tied to the positioning method. For instance, the cell identity method normally reports the position of the terminal with a polygon representing the cell extension whereas the round trip time (RTT) positioning method normally exploits the ellipsoid arc shape. The need for shape conversion functionality arises because the end user, to whom the determined position is reported, may prefer other position formats than generated by the applied positioning method. As an example, an emergency center may require that the location of the caller is presented as a circular confidence area at a given confidence level. However, the output of a positioning method can be in another shape, for instance a polygon, an ellipsoid, or an ellipsoid arc. Furthermore the provided shapes may correspond to different confidence levels.
In a WCDMA system, the need for a flexible shape conversion functionality is rapidly increasing due to the introduction of new positioning methods. It can be expected that cell identity positioning as well as full support of UE based A-GPS will be provided and an addition of RTT positioning can be expected. The reported confidence regions of these positioning methods are cell polygon (cell ID), ellipse with or without altitude (A-GPS), and ellipsoid arc (RTT). At the same time, operator reporting requirements over the service interfaces, e.g. RANAP or PCAP, often require reporting in terms of point with uncertainty circle or ellipse with a specified value of the confidence. The following positioning methods and corresponding output shapes are planned to be implemented:
Positioning method:Output shape:Cell ID positioningPolygon shapeCell ID and RTT positioningEllipsoid arc shapeUE-based A-GPS positioningEllipsoid point with ellipsoiduncertainty region
At the same time, the following reporting requirements are present from operators:                Report by means of a point with uncertainty circle (as required to be able to serve E-911 emergency positioning in North America);        Report by means of an ellipsoid area (as required to be able to serve 112 emergency positioning);        Report all shapes (in case of commercial location-based services).        